1. Field of the Invention
This invention relates to electrically propelled vehicles and in particular to means for indicating the actual state of charge in the drive batteries of the vehicle.
2. Description of the Prior Art
In U.S. Pat. No. 4,008,423 of Clinton C. Christianson et al, which patent is owned by the assignee hereof, an improved electrically propelled vehicle is disclosed having a direct current drive motor operated from a propulsion battery of the vehicle. The vehicle is adapted to be operated both at low speed and high speed by different modes of operation of the motor.
Robert F. Bourke, in U.S. Pat. No. 3,938,020, which patent is owned by the assignee hereof, discloses a charger circuit for an accessory battery in such an electrically propelled vehicle.
Clinton C. Christianson et al, in U.S. Pat. No. 3,958,173, which patent is owned by the assignee hereof, disclose a power converter employing non-saturating interphase transformer means for providing an output voltage having a level controlled in response to an external control signal.
In U.S. Pat. No. 3,946,299, owned by the assignee hereof, Clinton C. Christianson et al disclose a battery state of charge gauge including an improved circuit for detecting and indicating the actual state of charge of the storage battery of the vehicle. The gauge circuit includes means for calculating the open circuit voltage of the battery and a function generator simulating the characteristic curve of the battery internal resistance relative to the state of charge of the battery. The circuit further includes means for providing a signal proportional to the battery current. The various signals are combined to produce an output signal corresponding to the actual state of charge of the battery. The circuit includes means for effecting an expanded scale voltmeter functioning so as to provide facilitated indicating of the state of charge to the vehicle operator.
It has further been conventional to utilize coulometers for integrating the battery current over relatively long time intervals, such as a plurality of hours, so as to measure the ampere hours withdrawn from and returned to the battery during the motoring and recharging modes, respectively. Such prior art circuits do not provide an accurate determination of the state of charge of the battery as they do not effectively take into consideration variable factors, such as the rate of discharge and the like.
Another approach to the problem has been to utilize the open circuit voltage of the battery. This method of state of charge determination, however, presents a serious disadvantage of requiring that the indication must be made when the battery current is zero and, thus, may not be utilized during normal operation of the vehicle. Further, because of the relatively small change in terminal voltage corresponding to substantial change in the state of charge of the battery requiring the use of expanded scale voltmeters or the like, the prior art structures utilizing such state of charge determination means are relatively expensive and complicated.